Circular hole coming out oval

[bigo93]

New to 3D printing and have bought a CTC Makerbot dual clone.

First thing I have done to upgrade the printer is a larger print bed, by printing extenders.

But as you can see the holes have come out a little distorted and are more oval than circular.

Any ideas on how to fix?
Thanks


Used Makerware software to export file.
Printed on masking tape, was a little adhesion problems where the corners came off a little.
But this hole is in the middle of the print which stayed on the bed well.

Used ABS filament, bed at 100-110
Extruders at 260

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